Iron is known to enhance the production of the highly reactive and toxic hydroxyl radical, thus stimulating oxidative damage. Iron has been associated with a number of diseases, disorders and conditions because humans have no physiologic means of eliminating excess iron.
Hereditary hemochromatosis, a condition in which the body accumulates excess amounts of iron, is one of the most common genetic diseases in humans. In the United States, as many as one million people have evidence of hemochromatosis, and up to one in every ten people may carry the gene for the disorder. Hemochromatosis is characterized by lifelong excessive absorption of iron from the diet, with iron accumulating in body organs, eventually causing inflammation and damage. Serious and even fatal health effects can result, including cirrhosis of the liver, liver cancer, heart abnormalities (leading to heart failure), diabetes, impotence, and arthritis.
Clinical thalassemia (major and minor) are hereditary disorders characterized by defective production of hemoglobin, which leads to decreased production and increased destruction of red blood cells. With severe thalassemia, regular blood transfusions and folate supplementation are given, resulting in iron overload. Since iron is usually not ingested in large amounts, the body holds onto what it receives and has no way of ridding itself of any excess. Iron overload is therefore the leading cause of death among thalassemia patients in industrialized nations.
Patients with b-thalassemia major (TM) or refractory anemia (as in myelodysplastic syndrome) who receive frequent or regular red-cell transfusions, coupled with increased iron absorption due to ineffective erytropoiesis, develop iron overload rapidly. The toxicity of iron begins when its load exceeds the tissue or blood binding capacity to join a mobile intracellular or free nontransferrin-bound pool in the blood, the unbound iron accelerates hydroxyl radical formation resulting in peroxidative damage to cells. In the absence of chelation therapy, chronically transfused patients inevitably undergo progressive deterioration in pancreatic, hepatic and cardiac function, and usually succumb to life-threatening arrhythmias or intractable heart failure as a result of iron overload. This usually happens in the second decade of life in poorly or unchelated patients with thalassemia major.
Deferoxamine (DFO), a naturally occurring siderophore, chelating iron in a labile intracellular pool that is itself rapidly renewed from the storage pool, was introduced in early 1960s. The minimal absorption of DFO from the gastrointestinal tract and its short half-life in the blood necessitate a slow prolonged parenteral administration of the drug to achieve net negative iron balance as the prime goal of an effective chelation therapy. The expense and inconvenience of DFO has led to a search for an orally-active iron chelator, and deferiprone (L1) has been used in the last years for oral treatment of thalassemia major patients, but the risk of agranulocytosis mandates a careful evaluation of the use of this drug. Other orally active iron chelators have reached clinical trials in the past decade but their use in iron chelation therapy need more investigation. The identification of suitable, preferably orally, effective iron chelators for the treatment of iron overload diseases, disorders and conditions still remains an unsolved problem.
Neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease (AD), are neurodegenerative syndromes for which at present no cure is available. Both diseases are the most widespread neurodegenerative disorders. They affect approximately 0.5% and 4-8%, respectively, of the population over the age of 50 years, thereby, considering the still growing number of the elderly, forming an increasing economic burden for society. Therefore, development of an effective drug for preventing (neuroprotective) and treating neurodegenerative diseases is essential for the whole society.
Numerous studies including in vivo, in vitro and relevant animal models have shown a linkage between free radical production and neurodegenerative diseases and disorders, such as Parkinson's diseases, Alzheimer's disease and stroke as well as ALS, multiple sclerosis, Friedreich's ataxia, Hallervorden-Spatz disease, epilepsy and neurotrauma.
For this reason, 8-hydroxyquinolines and hydroxypyridinones have been proposed for iron binding as antioxidant-type drugs. Since iron accumulation in neurodegenerative diseases is a common feature, the inventors have shown previously that it has a pivotal role in the process of neurodegenration (Youdim, 1988). We (Gassen and Youdim, 1999) and others (Cuajungco et al., 2000; Sayre et al., 2000) have suggested on several occasions the development of iron chelators as therapeutic agents for Alzheimer's disease and Parkinson's disease.
In Parkinson's disease, the brain defensive mechanisms against the formation of oxygen free radicals are defective. In the substantia nigra of parkinsonian brains there are reductions in activities of antioxidant enzymes. Moreover, iron concentrations are significantly elevated in parkinsonian substantia nigra pars compacta and within the melanized dopamine neurons. Latest studies have also shown that significant accumulations of iron in white matter tracts and nuclei throughout the brain precede the onset of neurodegeneration and movement disorder symptoms. Indeed the accumulation of iron at the site of neurodegeneration is one of the mysteries of neurodegenerative diseases because iron does not cross the BBB. Where the iron comes from and why it accumulates is not known.
The etiology of Alzheimer's disease (AD) and the mechanism of cholinergic neuron degeneration remain elusive. Nevertheless, the chemical pathology of AD shows many similarities to Parkinson's disease: the involvement of increased iron, release of cytochrome C, increased alpha-synuclein aggregation, oxidative stress, loss of tissue reduced glutathione (GSH), an essential factor for removal of hydrogen peroxide, reduction in mitochondrial complex I activity, increased lipid peroxidation, and loss of calcium-binding protein 28-kDa calbindin, to mention a few. These similarities also include the progressive nature of the disease, proliferation of reactive microglia around and on top of the dying neurons, the onset of oxidative stress and inflammatory processes.
Oxygen free radicals have been shown to be associated with protein denaturation, enzyme inactivation, and DNA damage, resulting in lipid peroxidation of cell membranes, and finally the cell death in neurodegenerative diseases. One of the profound aspects of neurodegenerative diseases is the accumulation and deposition of significant amount of iron at the neurodegenerative sites. In AD, iron accumulates within the microglia and within the neurons and in plaques and tangles. Current reports have provided evidence that the pathogenesis of AD is linked to the characteristic neocortical beta-amyloid deposition, which may be mediated by abnormal interaction with metals such as iron. Indeed, iron is thought to cause aggregation of not only beta-amyloid protein but also of alpha-synuclein, promoting a greater neurotoxicity. This has led to the notion that chelatable free iron may have a pivotal role in the induction of the oxidative stress and the inflammatory process leading to apoptosis of neurons. Iron and radical oxygen species activate the proinflammatory transcription factor, NFκB, which is thought to be responsible for promotion of the cytotoxic proinflammatory cytokines IL-1, IL-6 and TNF-alpha, which increase in AD brains is one feature of AD pathology. This is considered logical since iron, as a transition metal, participates in Fenton chemistry with hydrogen peroxide to generate the most reactive of all radical oxygen species, reactive hydroxyl radical. This radical has been implicated in the pathology of cell death and mechanism of action of numerous toxins and neurotoxins (6-hydroxydopamine, MPTP, kainite, streptocozin model of AD). Furthermore, such toxins mimic many of the pathologies of neurodegenerative diseases (AD, Parkinson's disease and Huntington's Chorea), one feature of which is the accumulation of iron, but not of other metals, at the site of neurodegeneration.
Iron alone or iron decompartmentalized from its binding site by a neurotoxin, e.g. the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA), may induce oxidative stress and neurodegeneration, as evidenced in previous studies of the inventors in which intranigral administration of iron-induced “Parkinsonism” in rats and the iron chelator desferrioxamine protected the rats against 6-OHDA-induced lesions of nigrostrial dopamine neurons (Ben-Shachar et al., 1991). It has thus been suggested that treatment or retardation of the process of dopaminergic neurodegeneration in the substantia nigra may be affected by iron chelators capable of crossing the blood brain barrier in a fashion similar to the copper chelator D-penacillamine used in the treatment of Wilson's disease. This therapeutic approach for the treatment of Parkinson's disease can be applied to other metal-associated neurological disorders such as tardive dyskinesia, Alzheimer's and Hallervorden-Spatz diseases.
Stroke is the third leading cause of death in the Western world today, exceeded only by heart diseases and cancer. The overall prevalence of the disease is 0.5-0.8% of the population. Stroke is characterized by a sudden appearance of neurological disorders such as paralysis of limbs, speech and memory disorders, sight and hearing defects, etc., which result from a cerebrovascular damage.
Haemorrhage and ischemia are the two major causes of stroke. The impairment of normal blood supply to the brain is associated with a rapid damage to normal cell metabolism including impaired respiration and energy metabolism lactacidosis, impaired cellular calcium homeostasis release of excitatory neurotransmitters, elevated oxidative stress, formation of free radicals, etc. Ultimately these events lead to cerebral cell death and neurological disfunction.
Treatment of stroke is primarily surgical. Much effort is being invested in less aggressive therapeutical intervention in the search for drugs which are capable of restoring normal blood perfusion in the damaged area as well as drugs which are designed to overcome the above listed damaging events associated with cellular damage.
Oxidative stress and free radical formation play a major role in tissue injury and cell death. These processes are catalyzed by transient metal ions, mainly iron and copper. In the case of stroke, since vascular damage is involved, iron is available for the free radical formation, a process that could be prevented by iron chelators. Indeed, with lazaroides (21-amino steroids), known free radical scavengers, a significant improvement of local and global ischemia damages induced in animals has been achieved.
Iron chelators and radical scavengers have been shown to have potent neuroprotective activity in animal models of neurodegeneration. However, the major problem with such compounds is that they do not cross the BBB. The prototype iron chelator Desferal (desferrioxamine) was first shown by us to be a highly potent neuroprotective agent in animal models of Parkinson's disease (Ben-Schachar et al., 1991). However, Desferal does not cross the BBB and has to be injected centrally. Desferal also protects against streptozocin model of diabetes.
Free radicals in living organism are believed to be produced by the reaction of transition metal ions (especially copper and iron) with poorly reactive species such as H2O2, [O2.−], thiols, lipid peroxides. Antioxidant metal chelators, by binding free metal ions (especially copper and iron) or metal ions from active centers of enzymes of the defense system, can influence the oxidant/antioxidant balance in vivo, and hence, may affect the process of dopaminergic and cholinergic neurodegeneration and have great therapeutic potential against neuodegenerative diseases.
Iron accumulation in aging and the resulting oxidative stress has been suggested to be a potential causal factor in aging and age-related neurodegenerative disorders (Butterfield et al., 2001). There is increasing evidence that reactive oxygen species play a pivotal role in the process of ageing and the skin, as the outermost barrier of the body, is exposed to various exogenous sources of oxidative stress, in particular UV-irradiation. These are believed to be responsible for the extrinsic type of skin ageing, termed photo-ageing. (Podda et al., 2001). Iron chelators have thus been suggested to favor successful ageing in general, and when applied topically, successful skin ageing (Polla et al., 2003).
Iron is a factor in skin photodamage, not only in ageing, apparently by way of its participation in oxygen radical production. Certain topical iron chelators were found to be photoprotective (Bisset and McBride, 1996; Kitazawa et al., 1999). UVA radiation-induced oxidative damage to lipids and proteins in skin fibroblasts was shown to be dependent on iron and singlet oxygen (Vile and Tyrrel, 1995). Iron chelators can thus be used in cosmetic and non-cosmetic formulations, optionally with sunscreen compositions, to provide protection against UV radiation exposure.
Other diseases, disorders or conditions associated with iron overload include: (i) viral infections, including HIV infection and AIDS—oxidative stress and iron have been described to be important in the activation of HIV-1 and iron chelation, in combination with antivirals, might add to improve the treatment of viral, particularly HIV disease (van Asbeck et al., 2001); (ii) protozoal, e.g. malaria, infections; (iii) yeast, e.g. Candida albicans, infections; (iv) cancer—several iron chelators have been shown to exhibit anti-tumor activity and may be used for cancer therapy either alone or in combination with other anti-cancer therapies (Buss et al., 2003); (v) iron chelators may prevent cardiotoxicity induced by anthracycline neoplastic drugs (Hershko et al., 1996); (vi) inflammatory disorders—iron and oxidative stress have been shown to be associated with inflammatory joint diseases such as rheumatoid arthritis (Andrews et al., 1987; Hewitt et al., 1989; Ostrakhovitch et al., 2001); (vii) diabetes—iron chelators have been shown to delay diabetes in diabetic model rats (Roza et al., 1994); (viii) iron chelators have been described to be potential candidates for treatment of cardiovascular diseases, e.g. to prevent the damage associated with free radical generation in reperfusion injury (Hershko, 1994; Flaherty et al., 1991); (ix) iron chelators may be useful ex-vivo for preservation of organs intended for transplantation such as heart, lung or kidney (Hershko, 1994).
One of the main problems in the use of chelating agents as antioxidant-type drugs is the limited transport of these ligands or their metal complexes through cell membranes or other biological barriers.
Drugs with the brain as the site of action should, in general, be able to cross the blood brain barrier (BBB) in order to attain maximal in vivo biological activity. The efficacy of the best established iron-chelating drug, Desferal, in the neurodegenerative diseases, is limited by its ineffective transport property and high cerebro- and oculotoxicity.
8-Hydroxyquinoline is a strong chelating agent for iron, and contains two aromatic rings, which can scavenge free radicals by themselves. In our previous PCT Publication No. WO 00/74664, various iron chelators have been disclosed and their action in Parkinson's disease prevention has been shown. The lead compound, 5-[4-(2-hydroxyethyl)piperazin-1-ylmethyl]-8-hydroxyquinoline (herein referred to as VK-28), was able to cross the BBB and was shown to be active against 6-hydroxydopamine (6-OHDA) in an animal model of Parkinson's disease.
It would be very desirable to provide novel iron chelators that exhibit also neuroprotective activity and/or good transport properties through cell membranes including the blood brain barrier.